The speed and scale of modern manufacturing processes, coupled with changes to the nature and properties of many raw materials, have increased the range and number of applications where electrostatic charge can accumulate. Indeed, in any organisation that stores, handles or processes flammable liquids, powders, gases or vapours, there is every likelihood of exposure to the risk of static-caused ignitions within hazardous atmospheres.
It is accepted that each day through Europe, a static related incident causes a serious fire or explosion, whilst in the UK alone statistics suggest that some 50 such incidents occur each year. Apart from business interruption costs resulting from lost product, these incidents may cause serious injury to people and damage to plant and the environment, as well as the possibility of legal action and the attendant bad publicity for the organisation concerned.
In order to combat the build up and unwanted discharge of static electricity, plant and equipment (both fixed and mobile) is generally bonded and grounded (connected to earth), thus dissipating any charge before a dangerous level can be reached.
One measure which is commonly taken to achieve effective grounding is the use of grounding and bonding clamps designed to enable the user to achieve an effective connection to portable/mobile items of plant in order to safely dissipate static electricity. Such clamps commonly comprise spring-loaded jaws by which the clamp can be fastened to the equipment concerned, the clamp being connected to one end of a low-resistance cable, the other end of which is connected to earth.
Obviously, satisfactory operation of grounding clamps and the like depends on satisfactory electrical connection between the jaws of the clamp and the equipment to which it is applied. If the clamp is not properly applied, or if the equipment is coated with insulating material, eg paint or an insulating oxide, such connection may not be achieved. The clamp is then ineffective in discharging any accumulating electrostatic charge and there exists a danger of spark discharge, and the attendant risk of explosion.
Known grounding clamps serve only to connect equipment to a secure earth. They do not provide any monitoring capability to alert a user in the event that the connection to ground is not made, eg due to loose application of the clamp to the equipment or the presence of paint or other insulating covering. Circuit designs which do monitor the quality of a connection to earth are known, but are generally embodied in complex units which are not portable and hence of limited usefulness. Such units may also not be sufficiently protected to be located in hazardous areas.